Humans consider themselves as “crown of the creation,” since they feel to be guided by a breeze of the “divine spirit” that enables some insights into the scene of life and into basics of earth. However, with respect to their relationships on earth, humans are just members of the animal kingdom and thus endangered by the same agents of disease that threaten the life of animals. Since humans as predators are also members of the food chain on earth, they may also become infected with agents of diseases that are on or inside of animals that belong to the daily human food. Diseases due to such animal-based or animal-transmitted pathogens are called zoonosis. Such pathogens may belong to the groups of prions, viruses, fungi, bacteria, and animal parasites, which may interact in a broad spectrum of pathways. The main topics of this book—Blastocystis species—belong to these pathogens. Thus many pathways of transmission of the numerous above-cited pathogens will run identically or are at least very similar. Therefore it is worthwhile to throw a glimpse onto the, in general, already available transmission pathways of the agents of zoonotic diseases while giving definitions and showing important examples.